(copyright)2003 Entek International LLC. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR xc2xa71.71(d).
1. Technical Field
This invention relates to a battery separator for use in a flooded cell-type lead acid battery and to a method for making such a separator.
2. Background of the Invention
In a flooded cell-type lead acid battery, the positive and negative electrodes are separated by a battery separator. Such separators are formed of materials that are sufficiently porous to permit the electrolyte of the battery to reside in the pores of the separator material, thereby permitting ionic current flow between adjacent positive and negative plates, but not so porous to allow physical contact between the electrodes or xe2x80x9ctreeingxe2x80x9d of lead between adjacent electrodes.
The battery separator currently used by most flooded cell-type lead acid battery manufacturers includes microporous polyethylene. This type of separator is composed of an ultrahigh molecular weight polyethylene, a filler (typically amorphous silica), a plasticizer (typically a processing oil), and certain minor ingredients such as an antioxidant, a lubricant, and carbon black.
Microporous polyethylene separator material is commercially manufactured by passing the ingredients through a heated extruder, passing the extrudate generated by the extruder through a die and into a nip formed by two heated calender rolls to form a continuous web. The continuous web is processed by extracting a substantial amount of the processing oil from the web using a solvent, drying the extracted web, slitting the web into lanes of predetermined width, and winding the lanes into rolls.
Until recently, separators were manufactured as sheets that were placed between stacked electrodes. It is now more common to wrap a separator around the negative or positive electrode and seal the separator, which has a width greater than that of the electrodes in pre-wrapped condition. Sealing is performed along both edges to form an xe2x80x9cenvelopexe2x80x9d around each electrode. The enveloped electrodes are stacked, either against one another or in an alternating pattern with unenveloped electrodes. The resulting stack is then compressed and inserted into the cell compartment of a battery case. FIGS. 1 and 2 are top views of, respectively, an empty cell compartment 10 and an open-ended enveloped electrode 12 placed within cell compartment 10. While cell compartment 10 can be made of any material known in the art, the cell compartment shown in FIGS. 1 and 2 is made of polyethylene plastic.
Typically such separators are manufactured to have multiple xe2x80x9cmajorxe2x80x9d ribs extending from at least one planar face of the backweb of the separator. The major ribs function to provide proper electrode spacing and to provide a space in which free electrolyte can reside. The major ribs are typically formed parallel to the longitudinal edges of the backweb and generally have a height above the backweb that is greater than the thickness of the backweb. The ribs may be formed in one of several ways: (1) the ribs can be formed integrally with the backweb; (2) the ribs can be subsequently applied to the backweb as a bead of the same material as or different material from that of the backweb; or (3) the ribs can be formed by embossing the backweb.
However, because the major ribs have a greater volume than a portion of the backweb occupying the same planar surface area, more processing oil remains in the major ribs during the extraction step than that which remains in the backweb, resulting in undesirably increased electrical resistance. Further, separators having major ribs are more expensive to manufacture in light of the additional separator material that is incorporated into the major ribs.
One approach to remedying these problems entailed forming submini-ribs between the major ribs. This allowed separator manufacturers to reduce the number of major ribs and thereby decrease manufacturing costs. By reducing the number of major ribs, the amount of processing oil that remained in the separator after extraction was reduced, thereby decreasing the electrical resistance of the separator. Further, the submini-ribs facilitated the production of a thinner backweb with no loss in compression resistance or separator support. The use of a thinner backweb further reduced the manufacturing costs and the overall electrical resistance of the separator.
Separators also help to hold the electrochemically active paste on the wire grids of the electrodes during battery operation. Although separators serve to limit the amount of electrode paste that falls to the bottom of the cell compartment, operational vibrations of certain batteries (car batteries, for example) are so violent that a significant amount of paste naturally falls off the electrodes and pools on the bottom of the cell compartment. While on the bottom of the cell compartment, the electrode paste reacts with other chemicals that rest there, forming what is referred to as xe2x80x9cmud.xe2x80x9d Mud typically includes residual sulfuric acid, lead sulfate, PbO, and PbO2. The accumulation of mud in the bottom of the cell compartment causes battery failure because the lower portion of each electrode makes contact with the accumulated mud, causing the battery to short-circuit.
One attempt to remedy this problem entailed adding mud rests to the bottom of the cell compartment. As shown in FIGS. 1 and 2, mud rests 14 are U-shaped planar members that extend along and upward from the bottom surface 16 of cell compartment 10, thereby separating from the enveloped electrodes the paste residue that falls from the electrodes and pools on the bottom of cell compartment 10. Mud rests 14 typically extend across the battery from wall to wall, although they may be staggered.
Although the mud rests prevent the enveloped electrodes from contacting the mud, the mud rests are typically of a thickness that allows a single mud rest to lie adjacent to the separator backweb between two adjacent submini-ribs. Upon assembly or during operation, the sharp-cornered mud rests nib against the backweb, creating one or more wear areas that ultimately develop into one or more holes in the separator. Over time, these holes cause the battery to short-circuit.
Although major and submini-ribs and mud rests are integral parts of separator technology, their combined use in a single, economically efficient, operational battery has not been successfully achieved. This invention addresses the desire to fabricate a separator for use in a flooded cell-type lead acid battery having mud rests.
An object of the present invention is, therefore, to provide an economically efficient separator having (1) major ribs that provide the proper electrode spacing and a space in which free electrolyte can reside and (2) submini-ribs that reduce the manufacturing costs and the electrical resistance of the separator. The separator envelops an electrode and is placed into a cell compartment having mud rests that separate the mud from the enveloped electrode so that the incidence of battery failure is minimized.
The separator of the present invention includes multiple major and submini-ribs that are patterned such that the predominant web structure that rests against the mud rests at the bottom of the cell compartment are the submini-ribs. In most prior art separators, the backweb of the separator rubbed against the mud rests, which eventually resulted in the formation of a wear area, and over time, a hole in the separator. In contrast, predominantly the submini-ribs, which are thicker than the backweb, contact the mud rests in the present invention. Thus the formation of a wear area, and eventually one or more holes, in the separator is significantly delayed. Because increasing the number of submini-ribs and decreasing the number of major ribs results in a decrease in manufacturing costs, the placement and number of the major ribs is modified to provide a cost-effective, highly reliable separator for use in a flooded cell-type battery containing mud rests.